Summary: Production of a specific quantity of protective antibodies capable of neutralizing active toxin following immunization with toxoid vaccines containing a Diphtheria component is required for release of the vaccine lot. Currently, a second animal test is employed to evaluate the efficacy of the animal response to the test vaccine. This research has focused on replacing this second animal test with an in-vitro cell test. Results of the six laboratory collaborative study were presented in December, 1997 at the FDA Science Forum. These data were also reviewed in several international meetings, and plans to develop and implement a collaborative study continue to be discussed. Discussions about modifications of the STS proposed method are continuing. These discussions involve the amount of toxin used in the in-vitro assay, and the use of a hyper-immune horse serum vs. sera from a single immunization of guinea pigs.